I'd hardly call it "heavily" discounted. It's often only £100 cheaper than an R fare and £200 less than a D fare. They really start climbing when you get up to C and J fares. Often I, R and D fares all have the same restrictions but I-fares are just the ones they sell first.

Anyway, most of my F sectors on BA have been I-fares upgraded to the F cabin, even once saving money when no I-fare seats as such were available (cabin showing I0 during a sale).